Expression and segregation of genes encoding CryIA insecticidal proteins in cotton
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Epistatic and environmental effects on foreign gene expression in cotton (Gossypium hirsutum L.) could influence the breeding, stability and, in the case of pest resistance, efficacy and durability of the foreign gene. This study was undertaken to characterize the expression and segregation of two foreign cryIA genes in a range of insect-resistant cotton lines derived in three backgrounds. The cryIA genes encoded insecticidal proteins from the bacterium Bacillus thuringiensis spp. kurstakl. The transformed cotton lines, MON 81 expressing the cryIA(b) gene and MON 249 expressing the cryIA(c) gene, were crossed to 14 cotton isolines with five different insect-resistance traits. CryIA gene expression and variation were examined in terminal leaves of 2293 F2 progeny and subsequently in 12 F(2:4) lines [cryIA(b) only] in field experiments conducted at two locations in Texas. CryIA gene expression was variable and influenced by genetic and environmental factors. Site-of-gene-insertion and cotton-background effects were significant sources of variation for the cryIA gene. Significant epistatic and/or somaclonal effects increased plant-to-plant variation and caused cryIA gene expression to behave as a quantitative trait. Environmental effects, between and within locations and over time, decreased parent-offspring correlations of mean cryIA gene expression between individuals from the F2 and F2:4 generations. Gene dosage at the cryIA locus influenced insecticidal protein concentration in F2 populations with the cryIA(b) gene insect - homozygotes produced 14% more CryIA(b) protein than hemizygotes. The CryIA phenotype segregated as a simple, dominant Mendelian trait. However, non-Mendelian segregation occurred in some lines derived from MON 249. Expression of cryIA genes in cotton lines was influenced by one or more of the following: site of gene insertion, gene construct, background genotype, epistasis, somaclonal mutations, and the physical environment. These results indicate that appropriate evaluation and selection procedures should be used in a breeding program to develop new cotton lines with pest-resistant traits conferred by foreign genes. Moreover, that a practical backcross breeding program could be used to develop cotton cultivars combining one or more pest-resistant traits from foreign and native gene sources.
